6 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



scendant of both Ctenodipterines and Arthrodires, nor of either group 

 to the exclusion of the other. Since, however, it partakes of the char- 

 acters of both, community of origin is necessarily presupposed for all 

 three groups. 



4. Arthrodires and Ctenodipterines may be regarded as specialized 

 offshoots which diverged in different directions from the primal Dipnoan 

 stem ; and only the more generalized descendants of the original stock 

 have continued to survive until modern times. 



5. The primordial stock must have been autostylic, diphycercal, 

 without a secondary upper jaw and dentigerous dentary elements, 

 and with a Uronemus-like or Dipterus-like dentition ; characters wliicli 

 do not permit us to ascribe the ultimate origin of Dipnoans to the 

 Crossopterygii, but suggest rather a descent from Pleuracanthus-like 

 sharks. 



6. The recognition of Arthrodires as an order of Dipneusti precludes 

 their association with Ostracophores in any sense whatever. The recently 

 revived group of " Placodermata " is, therefore, an unnatural assem- 

 blage and should be abandoned. 



7. In the light of present information, progressive modifications 

 amongst early Dipnoans may be represented graphically after some such 



scheme as follows : — 



Neoceratodus 



Ctenodus 



Uronemus 



Phaneropleuron 



Scaumenacia 



Dipterus 



Titanichthys 



Coccosteus, Dinichthys 



Mylostoma, 



Dinomylostoraa 



Homosteus 

 Macropetalichtliys 



Primitive Ceratodonts 



With this statement of the general nature of the problem, our task 

 is to substantiate the claim in regard to the close structural agreement 



